In the art of modern high-speed papermaking, it is well known to continuously measure certain properties of the paper material in order to monitor the quality of the finished product. These on-line measurements often include basis weight, moisture content, and sheet caliper, i.e., thickness. The measurements can be used for controlling process variables with the goal of maintaining output quality and minimizing the quantity of product that must be rejected due to disturbances in the manufacturing process.
Similarly, in the production of plastics it is known that specific polymers can be identified by their characteristic absorption peaks when exposed to radiation having certain wavelengths. Indeed, such on-line measurements can be employed to detect the presence and concentrations of specific polymers in the production, disposable, or recycling of plastic articles.
The on-line sheet property measurements are often accomplished by scanning sensors that periodically traverse the sheet material from edge to edge. For example, a high-speed scanning sensor may complete a scan in a period as short as twenty seconds, with measurements being read from the sensor at about 50 milliseconds intervals. A series of stationary sensors can also be used to make similar on-line measurements.
In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric and water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is, in essence, a water removal, system. A typical forming section of a papermaking machine includes an endless traveling papermaking fabric or wire, which travels over a series of water removal elements such as table rolls, foils, vacuum foils, and suction boxes. The stock is carried on the top surface of the papermaking fabric and is de-watered as the stock travels over the successive de-watering elements to form a sheet of paper. Finally, the wet sheet is transferred to the press section of the papermaking machine where enough water is removed to form a sheet of paper. Papermaking devices well known in the art are described for example in Handbook for Pulp & Paper Technologists 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and Pulp and Paper Manufacture Vol III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking systems are further described, for example, in U.S. Pat. No. 5,539,634 to He, U.S. Pat. No. 5,022,966 to Hu, U.S. Pat. No. 4,982,334 to Balakrishnan, U.S. Pat. No. 4,786,817 to Boissevain et al., and U.S. Pat. No. 4,767,935 to Anderson et al. Many factors influence the rate at which water is removed which ultimately affects the quality of the paper produced. As is apparent, it would be advantageous to monitor the final paper product so as to, among other things, predict and control the dry stock weight of the paper that is produced.
It is conventional to measure the moisture content of sheet material upon its leaving the main dryer section or at the take up reel employing scanning sensors. Such measurement may be used to adjust the machine operation toward achieving desired parameters. One technique for measuring moisture content is to utilize the absorption spectrum of water in the infrared region. A monitoring or gauge apparatus for this purpose is commonly in use. Such apparatus conventionally use either a fixed gauge or a gauge mounted on a scanning head which is repetitively scanned transversely across the web at the exit from the dryer section and/or upon entry to the take up reel, as required by the individual machines. The gauges typically use a broad-band infrared source and one or more detectors with the wavelength of interest being selected by a narrow-band filter, for example, an interference type filter. The gauges used fall into two main types: the transmissive type in which the source and detector are on opposite sides of the web and, in a scanning gauge, are scanned in synchronism across it, and the scatter type (sometimes called “reflective” type) in which the source and detector are in a single head on one side of the web, the detector responding to the amount of source radiation scattered from the web.
Although it is most common to position IR moisture gauges in the more benign dry-end environment, similar gauges are also employed in the wet-end of the paper machine. The wet-end moisture gauges are typically located at the end of the press section or the beginning of the dryer section. Gauges in these locations are useful for diagnosis of press and forming sections of the paper machine, or for ‘setting up’ the web for entry into the dryer section.
Plastics films can be manufactured in a variety of ways. Typically, raw materials such as thermoplastics are fluxed into a rubber-like mass and then passed through a series of nips formed by a number of heated cooperating rolls to form a film, sheet or web of the specified thickness. In addition, different types of polymer films can be laminated together to form multilayer films. It is desired for process control to monitor the thickness of films produced.
Many of the current spectroscopic type sensors rely on non-versatile, non-generic techniques to detect various characteristics or constituents of paper and polymer products. These prior art techniques typically use passive bulk optic components such as beam splitters (either amplitude or dichoric) and individual fixed filters. As is apparent, one disadvantage is that these sensors cannot be readily reconfigured to detect different constituents. In addition, it is difficult to employ an optimum filter bandwidth or to move the filter's center wavelength to compensate for temperature induced wavelength shifts in a spectroscopic material feature. Prior art sensors also do not afford the versatility of permitting a switch from a first mode of operation in which the weights of various constituents are detected using spectroscopic techniques to a second mode of operation in which the coating thickness, for example, is measured using interferometric techniques. Specifically, with interferometric techniques, it is particularly useful to employ a wavelength diverse method that has the capacity of tuning over a certain wavelength range at a particular spectral resolution which will be dependent upon the optical thickness, which is a product of refractive index and physical thickness, for the particular coating of interest. Such versatility is not attainable with prior art sensors using fixed width and position filters. Finally, measuring numerous constituents in a flat sheet with prior art discrete filters entails a correspondingly large number of detectors and filters. Given that signals have to be split many times, signal strength will be adversely affected with a concomitant reduction in the signal-to-noise ratio.
Prior art gauges were generally hardware configured for a particular application. This entailed defining a number of channels, typically in the range from 2 to 12 or more wavelength channels each with a specific filter with its own specific center wavelength and spectral width. This process is labor intensive, costly and inefficient, and moreover, the gauges included many redundant components.